Method and system for displaying the amplitude distortions of a transmission channel

ABSTRACT

In order to display the amplitude distortions of a transmission channel, the amplitude distribution of a measurement signal of a known amplitude distribution is measured at the output of the transmission channel. Based on a given multitude of amplitude measured values that are measured and digitized in succession, the frequency of occurrence of the amplitude distribution is then determined and displayed in adjacently connected amplitude windows.

[0001] The invention starts out from a method according to the preambleof the main claim.

[0002] In order to measure the characteristics of a transmission channelwith regard to non-linear amplitude distortions, the amplitudedistribution of a measurement signal to be transmitted, which has anamplitude distribution that is known in theory, can be used. Deviationsfrom this ideal amplitude distribution can thus be attributed to theinfluence of the transmission channel. Limiting power output stages ofhigh-frequency transmitters and the like can be measured in this way forexample with regard to non-linear amplitude distortions.

[0003] The object of the invention is to present a particularly clearand for the user advantageous display of the measuring result of such ameasurement of the amplitude distribution.

[0004] This object is achieved starting out from a method according tothe preamble of the main claim by its characterizing features.Advantageous developments ensue from the sub-claims.

[0005] Due to the evaluation according to the invention of the frequencydistribution of the multitude of amplitude measured values that aremeasured in succession, with simultaneous matching data reduction anddisplay of the frequency distribution in frequency values combined togive amplitude windows, a particularly clear and rapidly evaluatabledisplay is created for the user of a corresponding measuring receiver,as shown in FIG. 3.

[0006] The method according to the invention is explained in greaterdetail below with reference to schematic drawings.

[0007] The successive amplitude measured values measured by a measuringreceiver for example at the output of a transmitter output stage to bemeasured are digitized via an A/D converter and processed further assuccessive digital values via a processor according to FIG. 1. For afirst representation, a predetermined number n_(max) of digital values,which correspond to a predetermined measuring time, is evaluated, inpractice 100000 successive measured values for example. These successivemeasured values are now sorted according to frequency, to do this therespective memory cell of memory A from S_(max)+1 memory cells with anaddress corresponding to the amplitude value of the measured valueevaluated in each case is incremented for each measured value AD (n)according to FIG. 1 (theoretically possible amplitude values: 0 toS_(max)). In the example according to FIG. 1, the amplitude values 4, 6,1, thus integer numerical values respectively, are measured insuccession, for example. As indicated by the arrows drawn, theirfrequency is counted in the related memory cells S=0 to S_(max), at theend following the evaluation of all n_(max) amplitude values theirfrequency distribution is thus stored in memory A. In practice memory Ahas for example 1024 successive memory cells.

[0008] A reference value, for example the root-mean-square value, thatis used in the final display as the reference value is then calculatedfrom these stored frequency values. For a logarithmic display scale, theroot-mean-square value is the most suitable. The reference value Ref iscalculated by way of the formula indicated in FIG. 1 from the sum of theamplitude frequency HAD, the number of amplitude stages S_(max)+1 of thememory A and the evaluated number n_(max) of amplitude values.

[0009] To simplify the display of the amplitude distribution of theoutput signal measured still further, the amplitude frequency values ofthe memory A above the reference value Ref are respectively combined andadded to give so-called amplitude windows F=1 to F_(max). The lower andupper limits of the amplitude windows F are respectively designated S₁and S₂. In the case of window boundaries that do not precisely meet themiddle between two amplitude stages of the measured values supplied bythe A/D converter, the value of the frequency of the amplitude stageconcerned must be distributed weightedly to two windows to increase theaccuracy of the result. Due to this combination of frequency values, theabsolute frequency AH (F) is finally obtained for each amplitude windowF and displayed. In the example shown according to FIG. 1, 20 adjacentlyconnected amplitude windows F for example are provided above thereference value of 0 dB calculated as the root-mean-square value, therange 0 to 1 dB being assigned to the first window, 1 to 2 dB to thesecond window, 2 to 3 dB to the third window etc.

[0010]FIG. 3 shows the display derived therefrom on a screen or due to aprintout by means of a printer. The 1-dB amplitude windows F are enteredin a horizontal direction, and the amplitude distribution, i.e. thenumber of measured values in the respective amplitude window relative tothe number of measured values recorded in total, in a vertical directionas bars. 1E-2 means for example that 10⁻², thus 1% of all measuredvalues, are located within this 1-dB window.

[0011] The crest factor, the calculation formula of which is indicatedin FIG. 1, can also be calculated easily from the greatest amplitudestage s of the memory A actually occurring and from the reference valueRef calculated as the root-mean-square value, and displayed.

[0012] A certain number z of measured values AD (n) is required for astatistically significant statement regarding the actual amplitudedistribution. In many cases, the stated multitude n_(max) is notsufficient for this, thus k_(max)=z/n_(max) measured values have to berecorded to attain the required measuring accuracy. To do this, a ringmemory structure of the length k_(max) is used according to FIG. 2.First the relevant absolute frequency AH (F) is ascertained for eachamplitude window F for a first multitude n_(max) of measured valuesaccording to the processing scheme according to FIG. 1 and entered inthe first memory cell row k=1 of the ring structure, to be precisetogether with the reference value likewise calculated according to FIG.1 and also together with the crest factor. Then the absolute frequencyis again ascertained for each amplitude window according to the schemeaccording to FIG. 1 for the second multitude n_(max) of measured valuesand entered in the second memory cell row k=2 etc. until the entire ringstructure is fully described by k_(max) elements. Following each entryin a memory cell row k, the measurement is shown on the display, takingthe measured values processed up to now into account. Following entry ink_(max), the first row k=1 of the ring structure is overwritten again,so that only the most up-to-date measured values are always processed.From the absolute frequency values stored thus in the ring structure,the cumulative frequency is calculated for each amplitude window andfrom this finally the relative frequency for each amplitude window,which frequency is then finally displayed according to FIG. 3. The ringstructure can consist for example of k=100 individual memory cell rows.The relative amplitude frequency for each amplitude window that isfinally displayed according to FIG. 3 is calculated here from the addedabsolute amplitude frequency for each amplitude window divided by thenumber of measured values that have occurred so far, as indicated by theformulae according to FIG. 2.

[0013] The crest factor can also be determined from the maximum of thecrest factors calculated hitherto and displayed, as likewise indicatedby the formula in FIG. 2.

[0014] Finally, the current percentage of the number of recordedmeasured values required in all for a statistically significantstatement can be displayed and corresponds to the number of entries kcurrently present in the ring structure divided by k_(max) according tothe formula likewise indicated in FIG. 2.

[0015] In order to compare the amplitude distribution measured, asrepresented in FIG. 3 by the vertical bars, with the theoretically givenknown amplitude distribution of the measurement signal used, the lattercan also be faded in in the display image, as indicated in FIG. 3 by thehorizontal short dashes for each amplitude window.

[0016] Any signal with a known amplitude distribution is suitable as ameasurement signal; preferably a signal that is also transmitted inoperation via the transmission channel to be measured is used, forexample a digital television signal processed according to the DVB-Tstandard.

1. Method for displaying the amplitude distortions of a transmissionchannel, characterized in that a measurement signal of a known amplitudedistribution is transmitted via the transmission channel, the amplitudevalues are measured at the output of the transmission channel, thesesuccessive amplitude measured values are converted into digital values,the amplitude values measuring range is divided into adjacentlyconnected amplitude windows, from a given multitude of the successivelyformed amplitude digital values the frequency of their occurrence in theadjacently connected amplitude windows is determined, and the amplitudefrequency values thus ascertained for each amplitude window aredisplayed by means of a display device.
 2. Method according to claim 1,characterized in that a reference value, in particular theroot-mean-square value, is ascertained from the amplitude frequencyvalues determined in all, and only the amplitude frequency values of theamplitude windows occurring above this reference value are displayed. 3.Method according to claim 2, characterized in that the quadraticallyweighted average (root-mean-square value) is calculated from the sum ofthe amplitude frequency values as the reference value.
 4. Methodaccording to one of the preceding claims, characterized in that firstthe frequency of occurrence of the respectively different integeramplitude measured values of the given multitude are determined, thenthe reference value is calculated and the amplitude frequency valuesdetermined above this reference value are combined and added to formgroups respectively, and these totals of the respectively combinedgroups of amplitude frequency values are displayed in the adjacentlyconnected amplitude windows.
 5. Method according to one of the precedingclaims, characterized in that the crest factor is calculated from thegreatest amplitude frequency value ascertained and from the referencevalue.
 6. Method according to one of the preceding claims, characterizedin that the amplitude frequency is ascertained for each amplitude windowrespectively according to claim 1 for several successive givenmultitudes n_(max) of amplitude measured values and the cumulativefrequency is ascertained therefrom and then the relative amplitudefrequency for each amplitude window is calculated and displayed. 7.Method according to claim 5 or 6, characterized in that the crest factoris also determined for several successive given multitudes of amplitudemeasured values and the relative crest factor is calculated via thecumulative frequency ascertained therefrom and displayed.
 8. Methodaccording to one of the preceding claims, characterized in that theideal amplitude frequency of the measurement signal is also displayed inthe amplitude windows at the same time.
 9. Method according to one ofthe preceding claims, characterized in that the current percentage ofthe momentarily evaluated number of given multitudes is also displayedat the same time.